An accelerometer is a device which senses acceleration, as well as shocks and vibrations, along or about an input or sensitive axis. One type of such an accelerometer is a mechanical resonating accelerometer which senses linear accelerations parallel to, or along, an input axis. If acceleration is to be sensed three-dimensionally, a triad of such accelerometers is arranged such that a first accelerometer senses acceleration along the x coordinate axis, a second accelerometer senses acceleration along the y coordinate axis, and a third accelerometer senses acceleration along the z coordinate axis.
A linear accelerometer typically includes a damped seismic mass which is positionally constrained by spring forces. In response to an acceleration, the seismic mass moves relative to its support and, when the acceleration ends, the seismic mass is returned to its initial position by the spring forces. The displacement of the seismic mass due to acceleration is converted into an electrical output by various types of transducers in order to produce a measure of the acceleration.
For example, in a potentiometric accelerometer, the transducer is a potentiometer having a resistance held in a fixed position with respect to a support surface. A wiper arm of the potentiometer is driven by a mechanical linkage connected between a seismic mass and the wiper arm. As the seismic mass moves in response to accelerations, the mechanical linkage moves the wiper arm over the resistance of the potentiometer to change the electrical output from the potentiometer. This change in electrical output provides an indication of the amount and direction of acceleration.
An inductive type accelerometer typically uses an inductance bridge sensitive to the motion of a seismic mass. As the seismic mass moves in response to accelerations, the seismic mass drives a ferromagnetic armature with respect to two inductive coils resulting in an increase of the inductance of one inductive coil and a decrease of the inductance of the other inductive coil. The difference in inductances between the two inductive coils provides an indication of the amount and direction of acceleration.
A strain gauge accelerometer includes a seismic mass attached to a strain gage which may be fabricated out of metal wire, metal foil or semiconductors. Servo accelerometers and piezoelectric accelerometers are also known. In piezoelectric accelerometers, a seismic mass is mechanically connected to a crystal material which may be comprised of quartz or of such ceramic mixtures as titanate, niobate, or zirconate.
A typical prior art mechanical resonating linear accelerometer generally utilizes at least one, and more often two, quartz beams and is a rather complex mechanical assembly. In such a quartz beam linear accelerometer, a quartz beam is caused to vibrate at a base frequency. The quartz beam converts its mechanical vibration into an electrical signal which has a frequency which tracks the frequency of the mechanical vibration. In the presence of acceleration, the vibration frequency of the quartz beam changes and this change in vibration frequency provides an indication of the amount and direction of acceleration experienced by the quartz beam accelerometer.
If the quartz beam linear accelerometer employs two quartz beams, the two quartz beams are generally arranged so that, in the presence of an acceleration along an input (i.e. sensitive) axis, one of the quartz beams experiences an increase in vibration frequency and the other quartz beam experiences a decrease in vibration frequency. The difference between these vibration frequencies of the two quartz beams provides an indication of the amount and direction of acceleration along the input axis. Quartz beams which are arranged in this push/pull manner benefit from common mode rejection wherein changes in vibration frequency of one quartz beam in response to such environmental factors as temperature and pressure are negated by equal changes in vibration frequency of the other quartz beam.
Quartz beam accelerometers have several disadvantages. For example, in assembling a quartz beam accelerometer, the quartz beams are typically bonded or glued between a support and a seismic mass thereby creating undesirable stresses and other problems resulting from thermal expansions. These stresses and problems adversely affect the performance of the accelerometer. Moreover, quartz beams normally have a high Q when operating in a vacuum. However, when used in an accelerometer, such quartz beams often are required to operate in a chamber where a level of gas pressure is usually maintained for the purpose of gas damping the seismic mass suspension structure. Unfortunately, this gas pressure also damps the resonating quartz beams which thereby decreases the Q, and, therefore, the stability, of the vibrating quartz frequency. (The quantity Q as used herein is a quality factor relating to the stability of a vibrating device; that is, Q is generally defined as one-half of the kinetic and potential energy stored in a vibrating beam divided by the energy lost by a vibrating beam per vibration cycle. If the energy applied by a force to the vibrating beam at a given point in time is equal to the total energy (i.e. the sum of the kinetic and potential energies of the beam) stored in the beam at that point in time, the vibrating beam has no loss; however, any difference between this applied energy and the total energy of the vibrating beam is the energy lost by the vibrating beam. Furthermore in response to acceleration, the frequency of the quartz beam can change by only approximately 10% of its base frequency through its useful range, i.e. a quartz beam having a base frequency of 40,000 Hz., for example, is limited to a 4,000 Hz. variation in response to acceleration.